In the quest for communications systems with an ever greater information-carrying capacity per unit of time, i.e., baud rate, modulation formats are used in which data signals modulate a pair of quadrature-related carrier signals having the same frequency. The frequency of the carrier signals which is modulated is commonly referred to as the intermediate frequency (IF) which may be transmitted or, in the case of radio systems is generally upconverted or translated to a higher frequency. Such modulation of phase quadrature carrier signals is referred to by a variety of names, such as quadrature amplitude modulation (QAM), phase shift keying (PSK), or amplitude and phase modulation (APSK). The information represented by the data signals is, of course, virtually limitless and can represent voice, video, facsimile and the like. In addition, the transmission channel propagating the modulated carrier signals is also not limited and, at present, may include air, wire, or lightguide.
In systems utilizing the above-described modulation formats, the data signals are recovered in the system receiver which utilizes a complex array of circuitry comprising demodulators, Nyquist filters for spectral shaping, threshold detectors, carrier and timing recovery circuits and distortion compensation apparatus, such as equalizers and/or cancellers. In order to reduce the cost and physical size of the system receiver, a technique of recovering data signals from quadrature-related carrier signals requiring less circuitry would be highly desirable.